As the seasons change, the world's millions of starlings migrate in giant flocks, clouding the skies with beautiful patterns—and leaving behind noisy squawks and showers of droppings.

Jan. 28, 2013 12:18 p.m. ET

A flock of starlings put on a display over the town of Gretna, Scotland, Nov. 11, 2012. The starlings visit the area twice a year, in February and November.
Owen Humphreys/PA Wire via Associated Press…

A flock of starlings flew over a field near the city of Netivot, Israel, Jan. 24.
Amir Cohen/Reuters…

The birds flew next to power lines in Netivot, Israel, Jan. 24.
Amir Cohen/Reuters…